The demand for reactors carrying out reactions in a controlled manner at high temperatures and high pressures, preferably in as short a time period as possible, has been continuously grown in the past decades. Such reactors has come to the fore, among others, due to the growing demand for fastness and low-level by-product production.
This led to a novel scientific field, simply called ‘flash chemistry’ in the English terminology. Reactors carrying out reactions falling into this field are the so-called flash reactors. The most important features of this field were summarized by J. Yoshida et al (see “Flash Chemistry: Fast Chemical Synthesis by Using Microreactors”; Chemistry—A European Journal 14(25), pp. 7450-7459, 2008). A distinctive feature of such devices is the rapid temperature control which can be achieved by means of a quick feedback, as well as efficient heat exchange and/or heat transport. As the residence time of a reagent mixture is short in such a reactor, these reactors are also capable of performing reactions that are more selective than what is conventional.
To perform a chemical reaction, selecting the temperature is critical for several reasons. Firstly, the rate of a chemical reaction increases by temperature (see e.g. the well-known Arrhenius relation—K. J. Laidler, Chemical Kinetics, Third Edition (1997), Benjamin-Cummings). Secondly, to initiate a reaction, it is required that the activation energy is transferred to the reaction mixture, mostly in the form of heat. However, the higher the temperature of the reaction is and/or the longer the compounds (initial reactants, products, solvent(s), further auxiliary substances and additives) participating in the reaction are exposed to high temperature, the more and the higher amount of undesired by-products and decomposition products appear in the reaction mixture. Further increase in temperature will also result in reaching the boiling temperature of the reaction mixture, which is undesired in chemical reactions carried out e.g. in continuous tubular reactors. Thus, optimizing the reaction temperature of a given reaction is of great importance when chemical reactions are to be performed.
In analytics, nebulization/atomization/spraying is a well-known technique to feed in samples, a great number of ways is known for its realization. One of its oldest and most wide-spread forms is pneumatic nebulization, in particular concentric pneumatic nebulization by making use of a high velocity gas, a so-called combustion gas. This is used most often in flame atomic absorption spectrometry, wherein the introduced gas (e.g. air, oxygen) supplies the combustion of the flame. In conventional pneumatic nebulizers, however, a problem occurs as too large substance and gas flows will blow out the plasma flame. In order to avoid this, the flow rates of both the sample and the combustion gases have been reduced. This has been achieved by reducing the inner diameter (0.2 mm) of the capillary of the nebulizer. As a result, however, the efficacy of nebulization has suffered a radical drop as well. Moreover, due to precipitation and subsequent plugging at the end of the capillary, solutions having the concentration of above 1% by weight became generally useless.
As the resulting primary aerosol exhibits a rather heterogeneous drop size distribution, a spherical collision body is placed in front of the nebulizer. By forcing the drops of said primary aerosol to collide with this body, said drops are getting broken up further, while larger drops simply get trapped. The thus resulting secondary aerosol is then passed through a concentrically mounted, radially oriented baffle plate with a conveying gas. As a consequence, larger drops get trapped again. The drop size distribution of the thus obtained tertiary aerosol will be about 5 μm.
Ultrasonic nebulization has been elaborated to increase further the efficacy of nebulization. Its core feature is the utilization of a generator suitable for and/or capable of producing ultrasounds with frequencies falling into the frequency range from 200 kHz to 10 MHz. Waves created at the liquid/gas interface by the oscillations of the generator induce aerosol generation. The average drop size distribution arising this way depends on the surface tension and the density of the liquid to be nebulized, as well as on the frequency of the ultrasonic source. Ultrasonic nebulizers can be divided into two groups. In case of a nebulizer belonging to the first group, the solution is guided onto a chemically resistant piezoelectric crystal, while in case of the other group, a medium that transfers longitudinal (or pressure-) waves is introduced into between the solution and the piezoelectric crystal capable of oscillating. Compared to pneumatic nebulization, ultrasonic nebulization results in a more uniform drop size distribution. Moreover, the physical properties of the drops can be controlled by varying frequency of the ultrasonic source.
Spraying, especially ultrasonic spraying is most frequently used as a possible technical means in the field of coating, wherein the uniform layer thickness of a few microns, reproducibility and productivity are of huge importance. When spraying is applied, mostly inorganic materials are applied to surfaces of different geometries. A similar technique is chemical vapor deposition (CVD) which also serves for coating surfaces, mostly by a given inorganic compound at high temperature and in vacuum.
International Publication Pamphlet No. WO2012/033786 discloses a solution, wherein one or more evaporating nonpolar substances are introduced into a pyrolysis deposition system in order to create a cadmium sulfide photovoltaic film. To this end, a solution is used that comprises—in dissolved state—cadmium, sulfur and at least one further selected substance. The further selected substance (alcohol) of the solution is nonpolar, evaporates faster than water, but its heat capacity is lower than that of water. Said mixture is arranged within the pyrolysis deposition system that comprises one or more spray heads and one or more heating devices. The desired layer is deposited onto a substrate whose distance from the spray head is adjustable and whose temperature can be controlled by the one or more heating devices.
According to said document, although a spraying/nebulizing method is carried out in the pyrolysis deposition system, no organic chemical reactions take place in this case as the applied process is substantially based on inorganic substances. Furthermore, no chemical conversion occurs when the procedure is completed.
U.S. Publication Pamphlet No. US20030230819 teaches a method for microencapsulating pharmaceutical ingredients with low molecular weight by means of ultrasonic atomizers. The applied apparatus comprises a coaxial atomizer, two liquid inlets and an ultrasonic generator. One liquid flows through an inner nozzle and the other liquid flows through an outer nozzle. Both flows pass through the same atomizing surface, wherein the mixture is broken up into micro droplets due to the vibration energy. The thus obtained particle size distribution is between 1 to 100 μm. The ultrasonic atomizer operates at low energies, and therefore it does not damage biological matter, e.g. blood, antibodies and bacteria.
Said process takes place at low temperature in the presence of two systems being mixed, the object is to get the active ingredient coated. No organic chemical reaction takes place under circumstances that are typical for pyrolysis. Furthermore, the ultrasonic spraying system has been used to achieve optimal mixing and particle size; the use of ultrasonic spraying to increase chemical activity is neither mentioned nor hinted at in said document.
International Publication Pamphlet No. WO2013/050402 reports on a device that is capable of producing organic metal-containing compounds and catalysts, named metal organic frameworks (MOF). Generally, a hydrothermal process is applied, wherein crystals are being grown slowly from a hot metallic (e.g. a metal salt) solution. As the crystals grow slowly and in a reversible manner, there is a high chance of forming defects therein. If this happens, the crystal has to be dissolved again, which results in crystals falling into the size range of milli- and micrometers. Said document also discloses a method wherein at least one metal ion and at least one organic ligand with a valence of two are being supplied into a spray dryer in the presence of a solvent. The mixture passes through a nozzle and the drops thus forming are being dispersed by hot gas. As a result, the reaction time required for the synthesis significantly reduces, dry crystals can be collected, and filtering and further processing steps can be avoided.
The production method, which has significantly improved efficacy compared to the previous methods, covers merely the temperature range of 80 to 200° C. Moreover, it applies no ultrasonic spraying unit, as the objects to be achieved do not include the small particle size distribution and the increase in the reactivity of the reaction mixture.
The spray pyrolysis technique is also known from applications used to dispose hazardous materials. In particular, U.S. Pat. No. 5,359,947 teaches a system for destroying packaged hazardous and toxic medical waste by means of molten metal heated to 800° C. The reactor comprises a two-part pyrolysis unit and a bottom outlet for the molten metal that leads from a first combustion chamber to a second one, into which the packages are fed. Glass objects melt on the surface of the molten metal, corrosion-resistant steel, e.g. the material of injection needles, and further metal objects get dissolved, organic materials get burnt and disintegrate into their constituents. The path of the pyrolysis products is heated to 250° C., and thus any pathogens and hazardous materials are destroyed in the described system.
In case of waste destruction, there is, generally, no need to control sophisticated reaction paths by means of changing the temperature, flow rate and other parameters. Thus, the use of spray pyrolysis in this field cannot be compared with the technique to be described in detail in what follows.